1. Field of the Invention
The present invention generally relates to a photographic zoom lens assembly and, more particularly, to the photographic zoom lens assembly of varifocal type.
2. Description of the Prior Art
A majority of photographic zoom lens assemblies now in use are of a standard design wherein the zooming operation does not affect the position of focus. In this design, the stroke of movement of the focusing lens group between the closest available position and the infinity position falls within the entire zooming range in which the zooming lens group can move to accomplish a change in focal length of the lens assembly as a whole. This means that the stroke of movement of the focusing lens group between those extreme positions remains constant regardless of a change in focal length that would result from the zooming operation. Therefore, a photographer need not go through a focusing operation each time the focal length is changed.
However, this standard design requires a highly sophisticated optical design that hampers a reduction in size and cost of the zoom lens assembly.
As an alternative to the standard zoom lens assembly discussed above, the varifocal lens assembly is well known in the art. For a given available zooming ratio, the varifocal lens assembly is generally considered having merit which is demerit of the standard zoom lens assembly. In other words, as compared with the standard zoom lens assembly having the same available zooming ratio, the varifocal lens assembly can be assembled light-weight and compact in size.
However, in the varifocal design, the amount of movement of the focusing lens group from the infinity position towards a certain lens position generally intermediate between the closest available position and the infinity position tends to increase as the zooming lens group is moved from a wide position, corresponding to the smallest available focal length, towards a tele position corresponding to the largest available focal length. Accordingly, if the stroke of movement of the focusing lens group is fixed to a value generally employed in the standard design, the minimum camera-to-object distance available to the varifocal lens assembly when the focusing lens group is set at the closest available position becomes larger as the focal length is increased. Conversely, if design is made that the minimum camera-to-object distance available to the varifocal lens assembly when the focusing lens group is set at the closest available position is relatively small even when the zooming lens group is set at a position of largest available focal length, the resultant varifocal lens assembly would be such that, when the zooming lens zooming lens group is set at a position of smallest focal length, the focusing lens group would extend a distance with which the optical performance can no longer be guaranteed, or which may result in an abutment with other lens groups and/or a stationary component part. Accordingly, in the varifocal lens assembly, it is a general practice to vary the stroke of movement of the focusing lens group with a change in focal length.
In an attempt to obviate the inconveniences inherent in the varifocal lens assembly, the Japanese Laid-open Patent Publication No. 63-261210, published Oct. 27, 1988, discloses a varifocal lens drive device which comprises a detecting means for detecting the position of a focusing lens group on the optical axis, a drive means for driving the zooming lens group, a calculating means for calculating the amount of extension of the focusing lens group from the infinity position towards the closest available position when the lens assembly is set at any focal length position, and a comparing means for comparing respective outputs from the detecting means and the calculating means with each other so that a detected end signal can eventually be provided for controlling the drive means.
Also, the Japanese Laid-open Patent Publication No. 62-284317, published Dec. 10, 1967, discloses a photographic zoom lens assembly including a zooming sensor installed within a lens barrel, and an external control circuit operable to refer to data representative of the range in which the focusing is possible, which range is variable according to the output from the zooming sensor and the selected focal length, thereby to control the focusing lens group not to depart from the focusing range.
The Japanese Patent Publication No. 1-19121, published Apr. 10, 1969, discloses a focusing system in a zoom lens assembly wherein the provision has been made of a regulating member for detecting a departure of the focusing lens group from the focusing range in dependence on the selected focal length and for subsequently driving the focusing lens group into the focusing range.
In any one of the prior art systems discussed above, an electric circuit system is employed for varying the stroke of movement of the focusing lens group and, in the event of an abnormality occurring in a control member such as, for example, an erroneous detection of the position of the lens groups, there is a possibility that the lens groups may collide with each other. Also, the use of an extra detecting member for the detection of the position of the lens groups is necessitated. On the other hand, where the lens assembly is of a type requiring an electric power supply from a camera body to which the lens assembly is mounted, the removal of the lens assembly from the camera body disables the electric circuit system built in the lens assembly and, therefor, the lens assembly still has the above discussed problems when the zooming operation is performed unnecessarily.